Monopulse radar system



Nov. 13, 1962 G. M. KIRKPATRICK 3,064,253

MONOPULSE RADAR SYSTEM Filed Aug. 5, 1957 2 Sheets-Sheet 1 AMPLITUDE INVENTOR, GEORGE M. KIRKPATRICK BY wm/m A 7' TORNE X Nov. 13, 1962 Filed Aug. 5, 1957 2 Sheets-Sheet 2 E -E l2 A a A MIXER H L.O. MIXER 2: IF A F AMPLIFIER AGE AMPLIFIER COMPARISON EN E CIRCUIT VOLTAGE c 22 1' ADD 1 v E E I A 151 24a SUBTRACT Z ADD DIFFERENCE RECTIFIER RECTIFIER 32g ELEVATION SUBTRACT Ecs: IEAI IEBI L IEA|*IEE 38 V V 40 LIMITER PHASE E DETECTOR AZIMUTH Ecs IN VEN TOR, GEORGE M. K/RKPA Tfi/CK.

A T TORNE X United htates Patent G 3,064,253 MONOPULSE RADAR SYSTEM George M. Kirkpatrick, North Swacuse, N.Y., assignor to the United States of America as represented by the Secretary of the Army Filed Aug. 5, 1957, Ser. No. 677,181 4 Claims. (Cl. 34316) This invention relates to radar tracking systems and more particularly to receivers adapted for use .in monopulse radar systems.

In one type of monopulse radar system known as the combination amplitude-phase comparison system, phase comparison is used in the azimuth plane and amplitude comparison is used in the vertical or elevation plane. Such a system includes two antennas placed side by side in a horizontal plane but tilted, one up and one down, in the vertical plane. With such an arrangement position err or correction voltage signals for targets at any point in the antenna beam are obtained in two dimensions. Since these position error correction signals, usually designated by the letters ECS, are a measure of the deviation of the target from the antenna boresight axis within the beam pattern, they should be independent of size and range of the target and also be linearly related to the target angle with respect to the boresight axis over a large portion of the antenna beam. For the combination amplitude-phase comparison system utilizing rectangular or square antenna apertures, it was found that when the elevation andazimuth error correction signals were derived from the ratio of the vector signal sum and difference antenna voltages, the linearity of the ECS in the amplitude comparison plane (elevation) was dependent on the phase angle separation of the antenna feeds along the horizontal axis. Because of this factor, elevation and azimuth error correction signals could not be used except for targets very closeto the boresight axis of the antenna. Another factor which previously limited the use of elevation and azimuth error correction signals for targets very close to the boresight axis was that due to the presence of cross-talk.

It is therefore a primary object of the present invention to provide an improved monopulse radar system receiver wherein the error correction information from target signals on the elevation axis is made independent of the azimuth axis.

It is another object of the present invention to provide an improved monopulse radar system receiver wherein the error correction signals are independent of size and range of the target and, in addition, are linear over a relatively large portion of the antenna beam.

It is still another object of the present invention to provide an improved monopulse radar receiver wherein the eifect of cross-talk in the elevation and azimuth error correction signal is greatly minimized.

Briefly, the present invention is directed to a means for deriving the elevation and azimuth error correction signals in a monopulse receiver system of the combination amplitude-phase comparison type wherein the two antenna radiation patterns are aligned in a horizontal plane but relatively displaced in a vertical plane. Included are means responsive to the combined antenna output signals for producing respective sum and difference vector signals and means for converting the sum and difference vector signals to respective sum and difference vector signals at a prescribed intermediate frequency. Also included are discrete amplifying means responsive respectively to the sum and difference vector IF signals.- In addition, there is included discrete means for respectively adding and subtracting the amplified sum and difference vector IF signals whereby the respective antenna output signals are 3,054,253 Patented Nov. 13, 1962 individually reconstructed, with the gain from the antenna outputs to the respective outputs of the adding and subtracting means being a prescribed value. Also included are means responsive to the respective outputs of the adding and subtracting means for deriving the absolute voltage value of the added sum and difference vector IF signals and the absolute voltage value of the subtracted sum and diiference vector IF signals. Included further are discrete means for respectively deriving the arithmetic sum voltage and arithmetic difference voltage of the absolute voltage values. In addition, there is included means in circuit with the discrete amplifying means and responsive to the difference between a prescribed reference voltage and the arithmetic sum voltage whereby the arithmetic sum voltage is maintained substantially equal to the reference voltage. The arithmetic diflerence voltage is the elevation error correction signal. The azimuth error correction signal is derived by detecting the difierence in phase between the vector IF outputs of the adding and subtracting means.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the antenna radiation patterns in the combination amplitude-phase comparison monopulse system;

FIGS. 1a and lb respectively illustrate the horizontal displacement and vertical squint of the antenna radiation patterns shown in FIG. 1;

FIG. 2 is a group of waveforms illustrative of the operation of the monopulse radar system and which is utilized in explaining the present invention; and

FIG. 3 is a block diagram of a monopulse system in accordance with the present invention.

The antenna feed arrangement utilized in a combination amplitude-phase comparison monopulse system is shown in FIG. 1. Antennas A and B are side by side in the horizontal plane and point directly ahead. This makes the antenna radiation patterns identical in the horizontal plane though they are slightly displaced horizontally as shown from the top view in FIG. 1a. The two antennas are also tilted, one up and one down, in the vertical plane to give the antenna pattern the vertical squint shown in the side view, FIG. 1b. The space patterns in elevation of the antenna feeds are shown in curves G and H of FIG. 2. A target in space will reflect the echo pulse and the amplitude of the echo pulse can be predicted according to these space patterns. The sum of curves G and H is the curve F which indicates the amplitude sum of the echo pulse on either side of the boresight axis. The difl'erence between curves G and H is shown in curve K, the negative amplitude portion indicating the phase re versal that takes place when the relative amplitudes at the antenna feeds changes on either side of the boresight axis. It will be seen that when an echo pulse is received from a point in the boresight axis, the amplitude of the difl erence signal is zero while the amplitude of the sum signal depends on the strength of the reflection. When an echo pulse is received from a point off the boresight axis but within the directive pattern, the sum signal still depends substantially only on the strength of the reflection. However, the amplitude of the difference signal depends both on the strength and on the deviation in the direction of the reflection from the boresight axis, and the phase relationship of the difference signal to the sum signal depends on the sense of the deviation; that is, the phases of the difference signal in response to a reflection from one side of the boresight axis is reversed with respect to that in response to a similar reflection from the other side of the boresight axis. By means of conventional duplex arrangements, antennas A and B are utilized for transmitting and receiving in a monopulse radar system. Assuming square or rectangular apertures for both feeds, it can be shown that the received vector quantities Such adder and subtractor circuits are so well known to the art that no further description thereof is believed necessary. If it is to be assumed that there has been no distortion of the vector signals at any point in the frequenat the antennas for a target at any point in the beam 5 cy conversion and amplification process, then the IF phase pattern are angle between the amplified E and E vectors will be Er the same as the original antenna signals. f (1) The IF sum and diiference signals 2E and 2E are applied to respective add and difference rectifiers 26 and =EF( /2 ie/z (2 28, the outputs of which provide the absolute voltage 2 value of the IF vector signal outputs from adder c1rcuit h 22 and subtractor circuit 24, respectively. The respective E zmaximum ignal t th receiver, absolljutedvoltage outputsdfrom rectgizers d26 and 28 age a/ 2 is the lag and lead in base of the signals at the two com me in a secon a er circuit an a secon su antenna feeds when the t arget is off the vertical axis of tfaetill lf to Produce fi p fi il if Signal; h system, equa to t e arit metic sum an arit metic i erence o e is the normalized elevation angle which is a function of the absolute Voltages derived from leetifiefs 26 and h h i l l ti angle d h antenna aper- As shown, the arithmetic sum direct-current voltage, is apture h i h plied as one input to a differential amplifier or cornis the normalized antenna lobe squint in elevation from Parisoh circuit 34 Where it is compared with a reference the antenna hotesight axis, direct-current voltage C applied to the other input of com- F 2, i h antenna pattern in h form of a dparison circuit 34. The output of comparison circuit 34 ct solution for h combined apertures, is adapted to provide a direct-current signal voltage prof(e eb) is one antenna pattern in the amplitude l portional to the dilference between the arithmetic sum tioh) comparison Plane, and signal voltage applied from second adder circuit and f(e+e is the other antenna pattern in the amplitude e refetehee Yoltage The tp t of C mparison cir- (eievatien) comparison plane cult 3 4 is applied to the input of an automatic gain con- Thus, for any target within the beam pattern the vector l tfi al g g lt lg g lg pg g i? g t h quantities E and E as defined in Equations 1 and 2 are P- an e p 1 15 t e applied to a monopulse radar receiver, shown in FIG. 3, 30 total galh from the Output of antennas A f n B to the through respective antennas A and B. outputs of adder circuit 22 and subtractor circuit 24, that Referring now to FIG. 3, the received vector signals t0 l l etg etlghlal i zg t t gg g E and E are simultaneousl cou led to a well known t eht e an metlc Sum tom a ell'eult y e w veguide hybrid comparator circuit 10 which is adapted Pressed J All'I BI With this input pp to to produce the respective Sum and diff of the Vector 35 automat1c gain control circuit 36 through comparison cirquantities E and E These relationships may be exchit 34, as h then the t t of e control Signal pressed as E +E =2, hereinafter referred to as the l l automatlc 52 lhi i l s l h i f i' z g sum vector signal, and E E =A, hereinafter referred t P 2 p 1 .2111 t e A aIhPi to as the difference vector signal. It is to be understood, W111 mathtam of course, that the sum vector signal 2 is proportional in [lE I-I-IE HGEC (5) magnitude to and has the sense of the algebraic sum of The arithmetic i f Signal is derived the amPhtlltte the g s antenn} feeds A and B from subtractor circuit 32 to provide the elevation error and that the difference vector signal A is p oport correction signal (Elevation ECS). The discrete vector magmtude to and h the sense P the htgehratc httetehce signal outputs of adder circuit 22 and subtractor circuit between the amphthde t the slghals 1H ahtel'lha feeds A 24 are also applied through respective conventional h The Sum Vector 51811812 t the dlfielehee Vector limiters 38 and 40 to a phase detector circuit 42, the signal A are each heterodyned w th the output from a output f i h provide h azimuth ESC local oscillator 12 n respective mixer circuits 14 and 16 To Show that the elevation ECS i independent f the to produce respectlve sum an dlfference IF veetor sazimuth axis, let it be assumed that the antenna vector t The Sum Vector slgnal p t from mlXeF signals E and E received by the antenna are as defined h 14 the dtfierehce IF vector slghat ph from in Equations 1 and 2. It can be seen that if the absolute mixer circuit 16 are applied to respective amplifiers 18 magnitude -I are taken, then the magnitudes t 29 labeled e the resileettve 0l1tP11t of [E and will not depend on the the relative phase of which are combined 111 a first adder circuit 22 and in a h vector signals E and E Also, the magnitude funcfirst subtractor clrcult Henee, the IF Output of adder tion of 0C, theazirnuth angle, is the same for both signals. ch'cutt 22 15 the vector Thus, if the ratio of the sum and difference of these (3) absolute magnitudes is taken, then the amplitude function may be cancelled out. As explained above, the output of and the IF output from subtractor circuit 24 is the vector th arithmeti difference circuit 32 is (|E ][E |)G difierehee and since A+ B A (4) G from Equation 5; It is to be understood, of course, that ZIF and AIF IEAI'HEBI signals remain dependent in magnitude and sense upon It can be seen that their parent signals. Thus, the discrete IF outputs from (IE l Ai Bi (6) adder circuit 22 and subtractor circuit 24 provide signal A B l Al+| B| vectors directly proportional in amplitude to antenna Now, with the values of E and E as derived from Equasignals E and E the original antenna signals, so that, tions 1 and 2, the ratio of the sum and difference of the in. effect, adder circuit 22 and subtractor circuit 24 serve absolute magnitudes shown in Equation 6 may be exto reconstitute the original antenna output vector signals. pressed as E r IEAHEE F( b)f( w 5 ,eb)f( +m n E E E, t Am L F(a/2,e,e )f(e-eQe +liEF(a/2,e,e )f(e+e )e gnu g g terms can be cancelled out, we have (i Al I BUG=ECS voltage Thus, the output from arithmetic difference circuit 32, []E ||E |]G, is equal to the ECS voltage and it is obvious from Equation 8, that for a given beam squint 6 the ECS amplitude is independent of azimuth and depends only on the elevation angle 6. Since the azimuth ECS is derived from the phase comparison of the E and E vectors in phase detector 42, it is obvious that the azimuth aXis ECS ratio is independent of the elevation axis.

While there has been described what is at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and and, since the modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

1. In a monopulse receiver system of the combination amplitude-phase comparison type wherein the radiation patterns from two antennas are aligned in a horizontal plane but displaced relatively in a vertical plane, means for deriving an elevation error correction signal and an azimuth error correction signal, said means comprising: means responsive to the combined antenna output signals for producing respective sum and difference vector signals, means for converting said sum and difference vector signals to respective sum and difference vector signals at a prescribed intermediate frequency, discrete means for respectively amplifying the sum and difference vector IF signals, discrete means for respectively adding and subtracting the amplified sum and amplified difference vector IF signals whereby the respective antenna output signals are individually reconstructed, the voltage gain from said antenna outputs to the respective outputs of said adding and subtracting means being a prescribed value, means responsive to the output of said adding means for deriving the absolute voltage value of the added sum and difference vector IF signals, means responsive to the output of said subtracting means for deriving the absolute voltage value of the subtracted sum and difference vector IF signals, discrete means for respectively deriving the arithmetic sum voltage and arithmetic difference voltage of said absolute voltages, means in circuit with said discrete amplifying means and responsive to the difference between a prescribed reference voltage and said arithmetic sum voltage whereby said arithmetic sum voltage is maintained substantially equal to said reference voltage, the arithmetic difference voltage being the elevation error correction signal, and means for detecting the difference in phase between the vector IF outputs of said adding means and said subtracting means to produce the azimuth error correction signal.

2. In a monopulse receiver system of the combination amplitude-phase comparison type wherein the radiation pattern from two antennas are aligned in a horizontal plane but displaced relatively in a vertical plane, means for deriving an elevation error correction signal and an azimuth error correction signal, said means comprising: means responsive to the combined antenna output signals for producing respective sum and difference vector signals, means for converting said sum and difference vector signals to discrete sum and difference vector signals at a prescribed intermediate frequency, discrete amplifiers for amplifying the respective sum and difference IF vector signals, discrete means for adding said sum and difference IF vector signals and for subtracting said sum and difference IF vector signals whereby they are produced discrete vector voltages proportional respectively to each of the output signals from said antennas, the voltage gain from said antenna outputs to the respective outputs of said adding and said subtracting means being a prescribed value, a first and second rectifier responsive respectively to the output of said adding means and the output of said subtracting means for deriving the absolute voltage of the added sum and difference IF vector signals, and the absolute voltage of the subtracted sum and difference IF vector signals, discrete means for respectively producing the arithmetic sum of said absolute voltages and the arithmetic difference of said absolute voltages, means in circuit with each of said amplifiers and responsive to the difference between a prescribed reference voltage and the sum of said absolute voltages whereby the sum of said absolute voltages is maintained substantially equal to the reference voltage, the arithmetic difference between said absolute voltages being the elevation correction signal, and means for detecting the difference in phase between the vector IF outputs of said adding means and said subtracting means to produce the azimuth error correction signal.

3. In a monopulse receiver system of the combination amplitude-phase comparison type wherein the radiation patterns from two antennas are aligned in a horizontal plane but displaced relatively in a Vertical plane, means for deriving an elevation error correction signal and an azimuth error correction signal, said means comprising: means responsive to the combined output signals of said antennas for producing respective sum and difference vector signals, means for converting said sum and difference vector signals to discrete sum and difference vector signals at a prescribed intermediate frequency, discrete amplifiers for amplifying the respective sum and difference IF vector signals, discrete means for adding said sum and difference IF vector signals and for subtracting said sum and difference IF vector signals whereby there are produced discrete vector voltages proportional respec tively to each of the output signals from said antennas, the voltage gain from said antenna outputs to the respective outputs of said adding and said subtracting means being a prescribed value, means responsive to the respective outputs of said adding means and said subtracting means for deriving the absolute voltage value of the subtracted sum and difference vector IF signals, discrete means for respectively deriving the arithmetic sum voltage and the arithmetic difference voltage of said absolute voltage values, a comparison circuit responsive to the difference between a prescribed reference voltage and said arithmetic sum voltage for producing a control signal, an automatic gain control circuit having a common output connection to the inputs of said discrete amplifiers and having its input responsive to said control signal whereby the said arithmetic sum voltage is maintained substantially equal to said reference voltage, the arithmetic difference between said absolute voltages being the elevation correction signal, and means for detecting the difference in phase between the vector IF outputs of said adding means and said subtracting means to produce the azimuth error correction signal.

4. The system in accordance with claim 3 wherein each of the absolute voltage'value deriving means comprises a rectifier.

References Cited in the file of this patent Worthington Dec. 24, 19 57 

